The present invention is generally in the field of forming and provides an apparatus and method for such forming. More particularly, the present invention relates to such apparatus wherein the forming force is generated by a rapid discharge of an electric pulse.
Metal objects can be formed to have a desired shape by a variety of processes. For example, metal in a liquid form, can be molded to have the desired final shape. However, such a process is applicable in certain specific cases and in addition requires the expense of a large amount of energy and complicated and costly installations for heating and cooling.
Metal has some pliability and accordingly metalwork pieces of one shape can at times be formed to have another shape. For example, a metal plate can be formed and cut to have a wide variety of desired shapes by using a mechanical press. Such pressing methods require a very costly and large installations required for achieving the necessary pressure for attaining the desired final object.
PMF is a process in which a metal workpiece or a portion thereof is put into a rapid motion by pulse magnetic fields which causes the workpiece to deform. One advantage of the PMF process is in that energy loss in this process is minimal and consequently there is no or very little heating of the workpiece. In addition, this process does not have the disadvantage of leaving tool marks, as is the case in a variety of other techniques. The PMF process uses a discharge capacitor or a bank of capacitors, a forming coil and often a field shaper, for creating an intense transient magnetic field. Very intense magnetic fields created in the PMF process, is a result of the rapid discharge of electric energy, stored in the capacitors, through the forming coil. The resulting eddy currents that are induced in the workpiece yields a magnetic repulsion between the workpiece and the forming coil, which cause the workpiece to deform.
A background on prior art apparatuses and methods for working of metal workpieces by the PMF process can be found in U.S. Pat. Nos. 3,654,787, 3,961,739, 4,170,887, 4,531,393, 4,807,731, 5,353,617 and 5,442,846, and in PCT Application Publication No. WO 97/22426.
Pulsed discharge forming (PDF) is a process whereby an electric spark or breakdown is discharged through a fluid particularly liquid which gives rise to plasma and/or vapor formation and this generates a shock wave within the liquid. PDF has been used for a variety of procedures requiring generation of an abrupt pressure wave for performance of work such as rock blasting, etc. Furthermore, PDF has also been applied for a variety of industrial processes.
When forming a shaped metal object from a planar metal plate, it is necessary at times to both shape the plate to require a desired three-dimensional pattern and to trim the edges so as to define the boundaries of the shape metal object.
It is an object of the invention to provide an apparatus and process for forming a metal plate into a dish having a desired three-dimensional pattern and shape.
The present invention provides an apparatus for forming a generally planar metal plate into a dish with a three-dimensional pattern, the apparatus comprising:
a mold having a forming surface with a contour corresponding to said three-dimensional pattern, and having edges corresponding to boundaries of the dish, which edges are defined by side walls essentially perpendicular to the forming plane;
a forming device comprising a fluid basin and pairs of electric discharge members within the fluid and having an opening facing the mold to allow transmission of a pressure wave from the fluid to the metal plate; and
an electric discharge circuitry for discharging a short and intense electric current through the pairs of electric discharge members generating an electric spark or breakdown within the fluid to yield formation of plasma, vapor or both.
The formation of plasma or vapor within the fluid basin generates a pressure wave which impacts on the metal plate causing it to deform and assume a three-dimensional pattern defined by the mold. In the process, peripheral portions are sheared along said edges of the mold.
The fluid in the basin is preferably a liquid, particularly an aqueous solution.
In accordance with another aspect there is provided a method for forming a generally planar metal plate into a dish with a three-dimensional pattern, comprising:
(a) providing a mold having a forming surface with a contour corresponding to said three-dimensional pattern, and having edges corresponding to boundaries of the dish, which edges are defined by side walls essentially perpendicular to the forming plane;
(b) placing the metal plate over the forming surface of the mold;
(c) providing a forming device comprising a fluid basin and pairs of electric discharge members within the fluid and having an opening facing the mold, the pairs of electric discharge members being connected to an electric discharge circuitry; and
(d) inducing said electric discharge circuitry to discharge a short pulse of an intense electric current through each of the pairs of electric discharge members, thus generating an electric spark or breakdown within the fluid creating a pressure wave in said fluid basin which impacts on and deforms the metal plate against said mold.
As will be appreciated, the sequence order of steps (a), (b) and (c) may be changed, e.g. to sequence (a)-(c)-(b) or (c)-(b)-(a) or (c)-(a)-(b), etc.
In accordance with an embodiment of the invention, the dish has a central concave depression, serving as a template for a central concave portion of the dish. The peripheral portion of the mold in this case may be generally planar thus defining a skirt portion of the dish. The peripheral portions of the mold may also comprise depressions, e.g. annular depressions, thus defining patterns in the skirt portions of the dish. A non-limiting example of a dish of this kind is that which is intended for use as an antenna, e.g. a satellite dish.
As a result of the pulsed discharge of current through the pairs of electric discharge members, portions of the metal plate are induced into a very rapid movement giving rise to either forming or shearing. Where the rapidly moving metal surface moves into a depression, any gasses which remain in the depression can resist the movement and prevent the obtaining of the desired shape defined by the mold, which is particularly problematic in the case of a large depression, e.g. in the case of the central concave depression of the embodiment mentioned above. Accordingly, in accordance with one preferred embodiment, gas ducts are provided to allow egression of gasses from one or more depressions in the mold. Preferably, such ducts are connected to a vacuum source, whereby all the gasses are removed prior to the generation of the pulsed magnetic force.
In accordance with an embodiment of the invention, the metal plate to be deformed is placed directly against the opening of the fluid basin. In accordance with another embodiment of the invention, the opening of the fluid basin is sealed by a flexible wall which transmits the shock wave to the metal plate.
In accordance with an embodiment of the invention, the apparatus comprises a planar forming coil member arranged so as to define a frame around the opening of the fluid basin for deforming a peripheral portion of the plate. Such a coil member is connected to a discharge circuit for discharging an intense electric current therethrough. For example, the coil member may be situated opposite the edges of the mold.
In accordance with an embodiment of the invention, each pair of discharge members consists of a pair of electrodes within the basin. In accordance with another embodiment, each pair of discharge members consists of two poles of a co-axial electrode. In accordance with yet another embodiment the pair of discharge members consists of the wall of the fluid basin and an electrode , whereby the electric discharge is between the electrode and said wall.
The apparatus may comprise any number of pairs of discharge members. For example, the apparatus may comprise two discharge members, three, four, six, eight, etc. In the case of a plurality of pairs of discharge members, all pairs may be connected to the same discharge circuitry, in which case current is discharged simultaneously through all electrodes; or alternatively, each pair may be connected to a different discharge circuitry. In the latter case, the discharge through all pairs may be simultaneous; or, preferably, the discharge may be timed to yield a predetermined discharge sequence.
As may be appreciated, the generated pressure wave impacting the metal plate yields a different force at different portions of the plate. In order to achieve an essentially uniform force over the entire surface of the plate situated at the opening, the electric discharge within the fluid may be generated essentially simultaneously from a number of pairs of discharge members distributed throughout the fluid basin. This results in that a number of pressure waves or an essentially uniform pressure front, which is a combination of individual pressure waves impacts the metal plate yield an essentially uniform forming force over the entire said portion. At times, however, it is desired to apply a forming force sequentially at different portions of the plate. This may be achieved, in accordance with an embodiment of the invention, by providing each of a plurality of pairs of electric discharge members with an independent electric discharge circuitry and then discharging an electric current pulse sequentially through different pairs.
The present invention further provides, by another of its aspects, a force generating device comprising a fluid basin with electric discharge members disposed therein which are connected to a discharge circuitry for discharging a rapid and intense electric current therethrough to generate an electric spark or breakdown between pairs of such members yielding pressure shock wave within the fluid; characterized in that the discharge circuitry comprising a capacitor battery connected at its one pole to at least one discharge member of a pair of such members and at its other pole to both one pole of a power supply and to one pole of a discharge switch; the other pole of the discharge switch being connected to the other discharge member of said pair and to the other pole of said power supply.
The above force generating device may, for example, serve as the forming device in any of the above apparatuses.
In the following, the invention will be described in a non-limiting manner with reference to the annexed drawings.